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Accretion Onto Black Holes from Large Scales Regulated by Radiative Feedback

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Accretion Onto Black Holes from Large Scales Regulated by Radiative Feedback Abstract Title of Dissertation: Accretion onto Black Holes from Large Scales Regulated by Radiative Feedback KwangHo Park, Doctor of Philosophy, 2012 Dissertation directed by: Professor Massimo Ricotti Department of Astronomy This thesis focuses on radiation-regulated gas accretion onto black holes (BHs) from galactic scales emphasizing the role of thermal and radiation pressure in lim- iting gas supply to the BH. Assuming quasi-spherical symmetry, we explore how the gas accretion depends on free parameters such as radiative efficiency, BH mass, ambient gas density/temperature, and the spectral index of the radiation. Our nu- merical simulations show an oscillatory behavior of the accretion rate, and thus the luminosity from the BH. We present a model for the feedback loop and provide analytical relationships for the average/maximum accretion rate and the period of the accretion bursts. The thermal structure inside the Str¨omgrensphere is a key factor for the regulation process, while with increasing ambient gas density and mass of BHs, eventually the accretion rate becomes limited by radiation pressure. The period of the luminosity bursts is proportional to the average size of the ionized hot bubble, but we discover that there are two distinct modes of oscillations with very different duty cycles, and that are governed by different depletion processes of the gas inside the ionized bubble. We also study how angular momentum of the gas affects the accretion process. In the second part of the thesis, we study the growth rate and luminosity of BHs in motion with respect to their surrounding medium. We run a large set of two-dimensional axis-symmetric simulations to explore a large parameter space of initial conditions and formulate an analytical model for the accretion. Contrary to the case without radiation feedback, we find that the accretion rate increases with increasing BH velocity v reaching a maximum value at v = 2cs;in, where cs;in is the sound speed of the photo-ionized gas, before decreasing as v−3. The increase of the accretion rate with v is produced by the formation of a D-type (density) ionization front (I-front) preceded by a standing bow-shock that reduces the downstream gas velocity to nearly sub-sonic values. Since the ionization front is beyond the classical Bondi radius for the hot ionized gas, in the BH frame of reference the accretion flow is similar to the stationary case. Interestingly, there is a range of densities and velocities in which the dense shell downstream of the bow-shock is unstable; its central part is destroyed and reformed intermittingly, producing a periodic accretion rate with peak values about 10 times the mean. This effect can significantly increase the detectability of accreting intermediate mass BHs from the interstellar medium (ISM) in nearby galaxies. For v > 2cs;in the central part of the bow-shock is not able to regenerate, the I-front becomes R-type and the accretion rate approaches the classical Bondi-Hoyle-Lyttleton solution. We find that the maximum accretion rate for a moving BH is larger than that of a stationary BH of the same mass, accreting from the same medium if the medium temperature is T < 104 K. This result could have an important impact on our understanding of the growth of seed BHs in the multi-phase medium of the first galaxies and for building and early X-ray background that may affect the formation of the first galaxies and the reionization process. Accretion onto Black Holes from Large Scales Regulated by Radiative Feedback by KwangHo Park Dissertation submitted to the Faculty of the Graduate School of the University of Maryland at College Park in partial fulfillment of the requirements for the degree of Doctor of Philosophy 2012 Advisory Committee: Professor Massimo Ricotti, Chair Professor Christopher Reynolds Professor Eve Ostriker Professor Richard Mushotzky Professor Tiziana Di Matteo (External) Professor James Drake (Dean's Representative) c KwangHo Park 2012 Preface The content of this thesis is drawn from three papers in a series published or to be soon published in the Astrophysical Journal. The main results of Park and Ricotti (2011) and Park & Ricotti (2012a) were presented as an oral dissertation presenta- tion titled Radiation-regulated Accretion onto Intermediate-Mass Black Holes at the 219th Winter Meeting of American Astronomical Society in Austin, Texas (2012). Below is the list of the papers, where the first two have been published while the third paper is waiting to be submitted (as of May 30th, 2012) to the same journal. • Park, K. and Ricotti, M., 2011, Accretion onto Intermediate-Mass Black Holes Regulated by Radiative Feedback. I. Parametric Study for Spherically Symmet- ric Accretion, ApJ, 739, 2. • Park, K. and Ricotti, M., 2012a, Accretion onto Black Holes from Large Scales Regulated by Radiative Feedback. II. Growth Rate and Duty Cycle, ApJ, 747, 9. • Park, K. and Ricotti, M., 2012b, Radiation-Regulated Accretion onto Black Holes in Motion. III. Bondi-Hoyle-Lyttleton Accretion with Radiative Feed- back, in preparation. Chapter 1 and 2 of this thesis are an introduction and numerical methods, re- spectively relevant for all three papers. Chapter 3 and 4 present the results in Park ii and Ricotti (2011) and Park and Ricotti (2012b) on accretion onto stationary BHs, while Chapter 5 presents the results for moving BHs in Park and Ricotti (2012b) in preparation. Summary and discusstion of the results from all three papers are given in Chapter 6. iii To my wife SueJean and our mothers iv Acknowledgements I thank God for opening my eyes to the beautiful universe and for guiding me all through these challenging steps. I can not thank Massimo Ricotti enough for being my academic parent, friend, and boss for the last six years. I am grateful to him for teaching me how to stand, walk, and run as a scholar. I also admit that I benefited from the wonderful members of my thesis committee members. I thank Richard Mushotzky for giving advice and supporting me for the last year of my graduate program and look forward to working on a project to- gether which will benefit from this dissertation. I thank Chris Reynolds for encouraging advice and scientific insight. I feel lucky to have Eve Ostriker in my committee before she leaves for Princeton and I thank her for constructive advice. I thank Tiziana Di Matteo at Carnegie Mel- lon University for being external committee member and I am looking forward to working together. Special thanks to James Drake from De- partment of Physics for being willing to serve as Dean's representative for my dissertation. v I thank wonderful faculties, staffs and graduate students at the Depart- ment of Astronomy. I thank Cole Miller for his kindness to graduate stu- dents and Doug Hamilton for listening to graduate students and giving advice as a graduate director. Special thanks to Stuart Vogel for his great leadership as a chair in the department. I also thank other faculty mem- bers Derek Richardson, Andrew Harris and Stacy McGaugh. I thank the department staffs, Mary Ann Phillips, Adrienne Newman, Eric McKen- zie, John Trasco, Mark Wolfire, and William Sebok for their endless support. Many thanks to Peter Teuben, Ed Shaya, Rob Olling, Tamara Bogdanovic, Soko Matsumura, and Roman Shcherbakov. I thank all the former and current graduate students, especially my academic sister Mia Bovill, Edmund Hodges-Kluck for correcting my papers, office mate Hao Gong, Rodrigo Herrera Camus, and the grand poobah Hannah Krug. I would like give my special thanks to my wife SueJean for joining the great journey with me, being with me, supporting me, and taking care of our kids. Truly, I would have not been able to finish my dissertation without her support. I'd like to thank our parents for their devoting support and sacrifices, especially during the births of my kids. Special thanks to my father-in-law YoungMoon Chae for advising me about doc- toral program. I am indebted to my two sisters SeungHui and JiYoung for their generous support and filling up the empty space in my family. I thank my kids MinJune and MinJee just for being there with us. I thank YoungWook Lee, YongChul Kim, YongIk Byun and Priya Natara- jan in the previous institutes for their support and advice. Special thanks to JungHyo Chae, Pedro Capelo, DongHun Park, ChunHo Park and Se- ungYong Lee. I am grateful to my friends in the church for praying for vi me and my family, and my friends at the tennis club KOSTAM for their great friendship and support. vii Contents List of Figures x 1 Introduction 1 1.1 Basic Definitions . .7 1.1.1 Bondi Accretion . .7 1.1.2 Luminosity and Radiative efficiency . .9 2 Numerical Simulations 13 2.1 ZEUS-MP and Radiative Transfer Module . 13 2.2 Radiation Pressure and Time Delay . 16 2.3 Axis-symmetric Accretion . 18 3 Spherically Symmetric Accretion 20 3.1 Qualitative description of accretion regulated by radiative feedback . 20 3.2 Comparison of 1D and 2D simulations . 25 3.3 Parameter space exploration . 26 3.4 Analytical Formulation of Bondi Accretion with Radiative Feedback . 33 3.5 Dimensionless accretion rate : hλradi .................. 34 3.6 Dependence on temperature at accretion radius . 37 3.7 Accretion rate at peaks and duty cycle: λrad;max, fduty ......... 40 3.8 Average period between bursts : τcycle ................. 41 3.9 Rayleigh-Taylor instability . 42 4 Growth Rate and Duty Cycle 45 4.1 Effect of Radiation Pressures . 46 4.1.1 Transition from Bondi-like to Eddington-limited Accretion . 47 4.1.2 Why is Continuum Radiation Pressure Negligible? . 51 4.2 Two Self-regulated Modes of Accretion : Collapsing versus Quasi-steady I-front .
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